As far as people who choose not to be aware of things on the astral realm, I've found that they're quite safe.
People have a natural shield that tends to protect them against the big baddies that lurk out there. The most that a lower-energy thing can do to these people is change their moods somewhat and make them feel ill.
The only real danger comes when people start going astral or doing spiritual thing, and treat it as a game. If it is a game, you can just stop playing, right? If you can just stop, then most people don't see a need to properly clean up after themselves, which weakens your natural shield. If you leave salt water on an iron bar, it's going to get weaker and weaker.
Also, when people first start to toy around with spiritual matters, they disregard their natural shield... To them, it is simply blocking the way, so they knock holes in it so that they can get out... Well, if you can get out, then something else can get in...
So, if you've never met them, and you aren't doing anything foolish, you don't have to worry about astral vampires. If you would like to learn more about astral travel, find someone who is fairly grounded to teach you.
